The present invention relates to a converter control device and, more particularly, relates to a converter control device which is connected between a first power supply and a second power supply; includes a configuration in which a plurality of converters each having a plurality of switching elements and a reactor and performing voltage conversion bi-directionally are connected in parallel; and changes the number of converter phases to be driven in response to converter passing electric power.
In a power supply system which uses a fuel cell, electric power is supplied by providing a voltage converter which steps up or steps down an output of a secondary battery and by connecting the voltage converter to output terminals of the fuel cell in order to cope with load fluctuations that exceed electric generation capacity of the fuel cell, to increase system efficiency, and to recover regenerative electric power when a motor capable of regenerating is used as a load. In such a case, the voltage converter is a converter having a function of direct current voltage conversion and is also referred to as a DC/DC converter; and, for example, a voltage converter composed of switching elements and reactors is used. Then, in view of reducing the rating capacity of the switching element, a plurality of converters are connected in parallel.
For example, Japanese Unexamined Patent Publication No. 2006-33934 discloses that, in order to cope with rapid changes in load volume that exceed the generating capacity of a fuel cell, a voltage converter that operates in a plurality of phases is connected between the fuel cell and a battery, and changes in the number of phases and the duty ratio of the voltage converter are determined by estimating a change in load volume. Then, it is disclosed that, generally, in a voltage converter provided with a plurality of phases, loss electric power lost in the converter fluctuates according to a value of passing electric power which corresponds to input and output conversion energy volume and operational work volume; when the passing electric power is large, loss of three phase operation in which a number of phases is more is smaller than that of single phase operation; and when the passing electric power is small, the loss of the single phase operation is smaller than that of the three phase operation. That is, the reason is described that the loss in a three bridge converter includes a reactor copper loss of a reactor coil, a module loss by operation of a switching element, and a reactor iron loss of a reactor magnetic material; the reactor copper loss and the module loss increase with an increase of the passing electric power and the reactor copper loss and the module loss in the single phase operation are larger than that in the three phase operation; and the reactor iron loss is not significantly effected by the passing electric power and the reactor iron loss in the three phase operation is larger than that in the single phase operation. Then, it is described that the single phase operation is performed in a region where the passing electric power is small; the three phase operation is performed in a region where the passing electric power is large; and voltage, current, and electric power are temporarily fluctuated in proportional-integral-derivative (PID) control because an effective value of an alternating current for the voltage conversion is fluctuated when changing from the three phase operation to the single phase operation; therefore the duty ratio is made to be raised temporarily and electric power shortage is compensated.
Furthermore, Japanese Unexamined Patent Publication No. 2003-235252 discloses a method of maximizing conversion efficiency in the case where a plurality of DC/DC converters is provided between an inverter and a battery. In this publication, it is described that a master slave DC/DC converter in which one of the plurality of DC/DC converters is set as a master DC/DC converter is provided; input electric power or output electric power of the master DC/DC converter is set as reference electric power; the number of the DC/DC converters including the master DC/DC converter to be operated is specified; an output voltage of the master slave DC/DC converter is then increased and decreased within a range not exceeding the maximum allowable charging voltage and the maximum allowable charging current of the battery to calculate conversion efficiency thereof; and the output voltage is adjusted so as to substantially coincide with the maximum conversion efficiency. In addition, it is described that the conversion efficiency of the DC/DC converter includes a primary switching loss and a loss depending on a forward voltage drop of a secondary rectifier diode; a primary loss increases at a time of high input electric power; and at a time of low input electric power, the primary loss decreases and a secondary loss becomes dominant.
Japanese Unexamined Patent Publication No. 2003-111384 discloses a method in which frequency of use of a specific DC/DC converter does not increase in the case where voltage of electric power of a main power supply is converted by a plurality of DC/DC converters connected in parallel and is supplied to an auxiliary battery. In this publication, it is described that each starting order of the plurality of DC/DC converters is made to be changed in accordance with a predetermined specified order; and as for the predetermined specified order, voltage-current characteristics of the respective DC/DC converters are measured to be set in accordance with contents thereof.
As described above, in the configuration which is used by connecting a plurality of converters in parallel, control which changes the number of converter phases to be driven in response to the passing electric power is performed. In such a case, the passing electric power can be obtained by calculation using a map or the like, for example. To cite one example, output electric power of a secondary battery to the converter is obtained from measurement values of an output voltage and an output current of the secondary battery, a load loss is subtracted therefrom, and calculation is made by multiplying conversion efficiency of the converter, and accordingly the passing electric power of the converter is obtained. However because in such calculation there is a calculation delay or calculation error, the passing electric power of the converter cannot be properly obtained and it is not sufficient to adequately make the change of the number of phases of the converter follow fluctuation in load. Furthermore, when driving a plurality of phases it is not preferable that the passing electric power of each phase fluctuates.